Did you update to v1.21 recently? It's known that the released version of v1.21 is not the best. v1.21 has random reboots and restarts. When I was using it it wouldn't reboot, it would just not let anything connect to the internet. DLink knows of this and has posted some beta versions that a few of us are using. I'm pleased to say that these versions have been performing very well over wired. My buddy is using a beta to and says it's working great, however told me today that for some reason, his xbox won't let him sign into XBL if his girlfriends is already online, his has to be signed in 1st. This is recent and an oddity.
Anyways, check out the DNS relay, not sure if this would make any difference. I'm also on Cable(one) and Beta 08 of 1.21 is doing great for me. I do have gameing setup for XBL and using GameFuel for the xbox. Dynamic Fragmentation is OFF.